Road Projects Benefit the Boise River

If you travel on State Street or Capitol Blvd. or frequent Traders Joes, Boise Brewery or other businesses on Broad Street, it’s been a season of lane closures, traffic delays and annoying inconvenience. You can take heart in your suffering – the Boise River will benefit from new stormwater management practices installed as part of these road projects.

Stormwater Pollutes the Boise River
stormwater discharge
Americana St. stormwater discharge. Photo Liz Paul

Stormwater is rain or snowmelt that flows off impervious surfaces like roads, parking lots, roofs and driveways and carries pollutants into the Boise River, including sediment, bacteria, nutrients, oil, grease, and heavy metals. The pollution can cause public health problems, promote growth of slimy algae, stress or kill bugs and fish, and generally gross river users out. For nearly a century, stormwater was piped to the Boise River, and while new discharges are prohibited, the old plumbing is still in use and polluted stormwater from older roads and neighborhoods still rushes to the river when it rains.

New Road Projects Capture Stormwater

There will be less polluted stormwater flowing into the Boise River this winter thanks to the installation of green stormwater infrastructure (GSI) by the Ada County Highway District (ACHD) and its partners. Green infrastructure uses vegetation, soils, and other techniques that allow natural storage, use and infiltration of stormwater in urban areas. Practices include biorentention ponds and swales, permeable pavers and concrete, planter boxes, rain gardens and green roofs.

Pedestrian improvement projects on State Street between Glenwood and Collister and between Willow Lane and Veterans Memorial Parkway include green stormwater infrastructure. Curbs, gutters, and sidewalks have been added, but instead of installing stormwater pipes to carry the water away, the runoff from State Street is directed via notches cut in the curb (curb cuts) to sandy areas where the stormwater soaks in. The sandy areas are two to three feet in depth and the sand removes sediment from the stormwater as it soaks into the ground.

roadside sand filters
Curb cuts and retention area on State St. Photo Liz Paul
Partnering for a Stormwater First

ACHD and Boise Parks and Recreation teamed up to install green stormwater infrastructure during construction of the Royal Blvd. extension on Capitol Street just south of the river.  Stormwater flows off the new roadway and parts of 9th St. and Capitol Blvd. into drop inlets in the gutters where it’s piped into three bioretention planter basins. The basins are 6 1/2′ wide and 4’deep. The two on the south are 25’long and the one on the north is 150′ long. It’s a first-of-its-kind project that will use a custom Bioretention Soil Mix and plants specially selected by ACHD to survive periodic floods of polluted stormwater. Species include maiden grass, hyssop, cone flowers, black-eyed susan and perennial grasses.

bioretention planter
Stormwater bioretention planter on Royal Blvd.
Cooperation Benefits the Boise River and the Economy

Stormwater management requirements for new development and major redevelopment in dense urban areas like downtown Boise can drive development to city edges and fuel sprawl. Thanks to a groundbreaking agreement between Boise and ACHD, developers will find it much easier to meet requirements and less polluted stormwater will discharge to the Boise River.

The new coooperative stormwater paradigm is playing out on Broad St. between 2nd St and Capitol Blvd. where ACHD, Boise and Capital City Development Corporation (CCDC) have reached agreement on installation and maintenance of stormwater facilities in the public right-of-way.

Broad St torn upTo you it just looks like a mess, but the carefully engineered infrastructure being built below the street and sidewalks is a radical departure that will enhance the Boise River.


Broad St. will be crowned to direct flow to permeable pavers in the parking lanes and to pocket bio-swales and infiltration planters. Stormwater from private property will discharge via roof drains to trees planted in special suspended pavement systems (Silva Cells) along the sidewalk. Capacity is being created to serve existing buildings without onsite retention and future development. This is known as “green turnkey” and will allow new developers to satisfy stormwater management regulations automatically.

The delays and detours are a small price to pay for a cleaner Boise River.